The present invention relates to a process and an apparatus for cutting various materials and more specifically pieces of wood with a view to obtaining thin products of varying thickness.
With reference to wood two methods are at present used, planar cutting and spiral unrolling.
The former method consists of splitting the wood by means of a perfectly ground wedge, called a splitter, by producing a planar displacement between them. It concomitantly comprises pressing the wood by means of a pressure bar which is suitably positioned in front of the splitter, this bar preventing an anarchic separation by maintaining the progression of the split in the intervention plane of the cutting edge of the said splitter.
During cutting the wood undergoes a deforming stress, the outer surface of the cut product being compressed, whilst its so-called open surface corresponding to the separation is distended. In the case of thin veneers (between 2/10 and 15/10 mm), this deformation is scarcely visible. However, it remains in existence and is generally accentuated during drying when the thickness is at least equal to 2 mm. Moreover, the tendency towards the product assuming a concave configuration is often aided by the presence of splits parallel to the fibres on the open surface.
This concave configuration of the cut products and the presence of splits represent serious disadvantages which can even compromise the use of these products.
The unwinding method comprises splitting and pressing as hereinbefore but in this case the piece of wood is rotated and the splitter and pressure bar are moved radially, being positioned tangentially in order to act. Thus, during unwinding a spiral split having a constant pitch is produced.
The wood undergoes a deforming stress comparable to that noticed during cutting. However, this stress is less accentuated but evolves during winding as a function of the decrease in the radius of curvature. There is still a tendency towards a concave configuration and it is not constant over the entire length of the unwound product.
Moreover, the larger the radius of curvature of the piece of wood the more easily thick veneers can be unwound. Thus, it is much more difficult if not impossible in numerous cases to develop a thick veneer from a small radius unwinding without cracks occurring on the open surface.
However, these cracks, whose number and depth vary constitute surface gaps impairing the appearance, reducing the strength of the product and aiding the tendency towards a concave configuration.
Finally, unwinding does not make it possible to develop the whole piece of wood. To bring about the driving of the piece, it is in fact necessary to leave a residual core or the like, the latter being larger proportionately with the length of the piece, proportionately with the greater thickness of the unrolled products, proportionately with the greater resistance of the wood, etc.
A special unwinding machine is described in German Patent No. 932,160, but would not appear to have been exploited. This machine comprises a rotor equipped with a knife and a pressure bar passing in front of the pieces of wood which extend to the outside (from the side opposite to the rotation axis), being moved towards the said rotor. The cutting radius is therefore constant but the end of the piece of wood during cutting is concave and the corresponding surface of the cut product convex. Thus, this product once again has a concave configuration and this is accentuated under the action of both the pressure bar and the cutting stress. Moreover, the said product inevitably has cracks which tend to open when an attempt is made at straightening. Finally, the cutting radius cannot be regulated.